In general, when the power switches are located on the periphery, analogue control of the supply voltage is employed, and when they are distributed within the circuit, digital control is employed. In any case, it is useful to limit the current that flows through the power switches. In particular this current should not exceed a maximum value, called the switch-on current, in order to preserve the integrity of the external power supply of the circuit, but also the integrity of the metallization levels of the circuit. It is therefore necessary to control the current delivered by the switches when the circuits are being powered.
In order to control the power supply of the circuit digitally, several groups of power switches are provided for beforehand, during the design phase of the circuit. Next, during operation of the circuit, groups of switches are turned on in succession in order to supply power to the circuit. A first group of switches is turned on first and then, in succession, other groups are turned on. Each turn-on control signal is delivered after a time delay in order to increase the supply voltage gradually. At the end of this power supply process, all the switches are turned on and the voltage delivered to the integrated circuit equals the external supply voltage. Moreover, the time delays are defined during the design phase of the circuit, but such a power supply method does not take into account circuit operating parameters such as temperature variations or the value of the external supply voltage. Since these parameters are not known during the design phase of the integrated circuit, wide operating margins are thus provided during the design phase, thereby making digital control of the power supply of the circuit imprecise.
Currently, integrated circuits possess several parts that do not necessarily operate simultaneously. For example, in a mobile phone the camera function may be off while the music player is on. It is therefore useful to be able to turn off, and then turn back on, part of an integrated circuit, as required.
Currently, integrated circuits are provided with power switches dedicated to supplying power to the various parts of integrated circuits. These switches may be located on the periphery of the circuit to be powered, or they may be distributed within the circuit, i.e. incorporated in the standard cells of the circuit.